r-gui 1.64 (new formula)

New formula for the R Mac GUI.

The following restrictions hold (based on the `INSTALL` instructions):
> Only Mac OS X 10.6 and higher are supported, and only 64-bit R

R.app is linked against the installed version of R (in opt).

HEAD version works via SVN repo (employs a minor hack due to the build
scripts requiring the SVN version).

Closes #831.